A multi-cell and multi-user Multiple Input Multiple Output (MIMO) system refers to a system composed of a plurality of cells and a plurality of pieces of User Equipment (UE), a Base Station (BS) being located in each cell. In this system, each BS and each UE may include multiple antennae, and each BS simultaneously serves multiple users by using the same frequency resource. Compared with a single-user MIMO system, the multi-user MIMO system may effectively improve the system capacity, is regarded as one of key technologies for the next generation wireless communication accordingly, and gains an attention from a standardizing organization such as a 3rd Generation Partnership Project (3GPP).
However, in downlink transmission of the multi-cell and multi-user MIMO system, when a certain BS (transmitter) sends information to users (receiver) in a cell where the BS is located, as a frequency reusing factor of the system is 1, the BS will cause an interference to users in other cells, the interference being an Inter-Cell Interference (ICI); and meanwhile, as the BS simultaneously serves multiple users by using the same frequency resource, an Inter-User Interference (IUI) will exist between multiple users in the cell. The simultaneous existence of the ICI and the IUI causes serious limitations to the performances of the system due to the interference. Thus, an effective interference suppression technology is the object of a prolonged endeavor of each research institute. An interference alignment technology is a novel potential interference suppression mode which has emerged recently, and the technology may greatly improve the system capacity. The interference alignment technology evaluates the system capacity by using a Degree of Freedom (DoF), and the evaluation accuracy is improved with the increase of a Signal Noise Ratio (SNR). The emergence of the interference alignment technology creates a new direction to interference suppression, the interference alignment technology may be applied to a system configured with a single antenna or multiple antennae, the complicated structural design of a terminal is not needed, it is unnecessary to modify a conventional network framework and protocol interface, and inter-cell interference suppression may be realized under the condition that data is not shared by utilizing the channel reciprocity of a Time Division Duplex (TDD) system. Consequently, the interference alignment technology receives a wide attention from all national and international academic institutes and research institutes.
A basic interference alignment idea refers to designing a pre-coding matrix at a transmitter, limiting an interference signal into a specific receiving signal sub-space and reserving another interference-free signal space for data transmission. However, on the basis of the interference alignment technology, the transmitter needs to obtain global Channel State Information (CSI). In the downlink transmission of the multi-user MIMO system, a user may feed CSI of a useful channel and an interference channel back to a BS serving the user via a feedback link, information is shared between the BSs via an X2 interface, and data transmission is performed. In the multi-user MIMO system, an appropriate pre-coding matrix is designed by using the interference alignment technology to eliminate the interference, all users need to feed complete CSI back to all BSs, the CSI is not subjected to any processing, the needed capacity load of the feedback link is heavier, and the spectral efficiency of the system is influenced to a certain extent. Thus, if the interference alignment technology is applied to the multi-cell and multi-user MIMO system, there are a lot of problems needing to be solved in a feedback process that the BS obtains the CSI by feedback.
In a single-user MIMO system having M transmitting-end antennae and N receiving-end antennae, there is only one user in each cell correspondingly, a BS i in a cell i only serves a user in the cell i (at this time, serial numbers of cells, serial numbers of BSs in cells and serial numbers of users in cells are i), and in order to eliminate the interference via the interference alignment technology and transmit a useful signal, it is necessary to satisfy the following two criteria:(Rj)HHjiTi=0,∀i≠j   (1)rank((Rj)HHjjTj)=dj   (2)
where, Hji represents an N*M-dimension channel matrix (namely CSI) from a transmitter i to a receiver j, dj is a degree of freedom capable of being obtained by a user j and is less than min(N,M), Rj represents a receiving elimination inference matrix of the receiver j, and Tj represents a transmitting pre-coding matrix of the transmitter i corresponding to the served receiver. Formula 1 represents that the interferences of all interference sources are aligned to the same direction and is eliminated from the receiver j, it is necessary for each transmitter to cooperatively design the transmitting pre-coding matrix in order to align the interferences to the same direction, and when the transmitting pre-coding matrix is designed, the following conditions shall be satisfied: span(HjlTl)= . . . =span(HjkTk)= . . . =span(HjkTk)(j≠k), where span(A) represents a space expanded by a matrix A; and Formula 2 represents that it is necessary to transmit the useful signal by sufficient dimensionalities so as to obtain a DoF desired by a user j. According to the above analysis, it may be seen that the most important thing in the interference alignment technology is that the transmitter needs to know CSI, that is, a channel matrix H, estimated by each receiver.
In an interference alignment research on the single-user MIMO system, the transmitter obtains CSI at a Transmitter (CSIT) in a global CSI feedback mode. FIG. 1 is a diagram of global CSI feedback of a single-user MIMO system.
In the feedback process, suppose each user broadcasts complex coefficients in an own channel matrix H to all interference sources in case of no errors, each user needs to feed N*M elements in the channel matrix back to all (K−1) interference sources, the magnitude thereof is (K−1)*N*M, the feedback information magnitude of the whole system is K*(K−1)*N*M, and the feedback magnitude is calculated only under the condition that each transmitter serves only one receiver; and in the multi-cell and multi-user MIMO system, if each BS may simultaneously serve multiple users, the feedback magnitude will be higher under this circumstance. Thus it may be seen that after the system is applied to the interference alignment technology, a throughput gain capable of being obtained is linearly related to the number of users. However, an overhead brought by feedback is linearly related to the square of the number of users, which may be influenced by the number of cells, the number of cells and the number of transmitter antennae simultaneously in the multi-cell and multi-user MIMO system. Thus, from a global perspective, when the number of users is very large, the performance loss of the system caused by the feedback of the complete CSI overhead of the whole system probably exceeds benefits capable of being obtained in terms of throughput increase.
In the downlink transmission of the multi-user MIMO system, each user only feeds CSI which is not subjected to any processing back to all BSs, and the CSI which is fed back may contain redundant information, which causes that the feedback overhead is higher. In view of that the information fed back by the user contains CSI of a served base station and further includes CSI of a base station causing an interference thereto, and redundancy may be eliminated in a mode of designing a pre-coding matrix by means of the latter. From a user perspective, the disclosure will appropriately group users, establish an appropriate feedback mechanism, and reduce the feedback overhead, thereby practically applying the interference alignment technology to the multi-cell and multi-user MIMO system. To sum up, the disclosure focuses on an interference alignment technology research based on a user grouping feedback mechanism in the downlink transmission of the multi-cell and multi-user MIMO system, wherein the determination of the user grouping feedback mechanism depends on a user cooperation processing policy. It is desired that the system obtains an optimal reusing gain in the multi-user MIMO system by using the interference alignment technology and the problems about the feedback overhead and the resource utilization rate of the system in the interference alignment technology are solved.
On the basis of the above problem analysis, the disclosure will design a CSI feedback mechanism from a perspective of user cooperation processing, and will propose a partial interference alignment solution based on user grouping in the downlink transmission of the multi-cell and multi-user MIMO system. By utilizing the solution, the problem about the feedback overhead of the system caused by application of the interference alignment technology to the downlink transmission of the multi-cell and multi-user MIMO system is solved.